The invention generally relates to the protection and control of electric motors and, more particularly, to a motor control device which provides ground fault detection for an electric motor.
When operating as a motor starter, a motor device functions to start, accelerate, and stop an electric motor by causing the various windings of the motor to be connected to, or disconnected from, a source of electric power in response to manual or automatic commands. To provide a desired sequence motor function based upon external inputs, motor starter logic can be implemented in conjunction with the motor starter. In addition to controlling the connection of power to the motor with contactors, the starter typically provides annunciator functions, such as visible status indication in the form of pilot lights. Each input and output, is generally binary in nature.
Motor starter logic can be implemented with electromechanical relays wired together according to a particular application. Relays, however, are large and relatively expensive. Consequently, logic circuitry using solid-state components have replaced wired-relay logic in some applications. Both of these techniques are still widely used, but are relatively inflexible. In the case of wired-relay logic, different applications may require expensive labor-intensive operations, such as rewiring and repositioning the relays. In the case of solid-state logic, different applications may require either modification of a circuit or a change in logic. In both cases, the lack of flexibility forces the manufacturer of the starter to build and stock different versions of starters for different applications. Each version may differ from another in the quantity, layout, and interconnection of components. Different manufacturing requirements for each version frequently result in problems of quality control. It is inconvenient, difficult, and costly to control the quality of many different versions of a motor starter.
The problem of inflexibility has been approached with the use of programmable devices. Programmable logic controllers (PLCs) are devices, usually containing a digital processor and a program memory, designed to make decisions based on successive single-bit information. The program steps stored in memory replace the combinatorial logic of solid-state and the wired logic of relay-type starters. Each input and output may be programmed to perform a particular control function. As the application changes, the generic inputs and outputs can be reassigned and the sequential steps reprogrammed. One problem with PLCs, as applied to the function of motor starting, its that the cost of a PLC is difficult to justify in simple starter applications requiring only a few inputs and outputs.
In addition to PLCs, relatively low-cost programmable integrated circuits designed for use in simple starter applications are available. One problem with these circuits is that the necessary supporting circuitry elevates the cost to unattractive levels relative to the cost of wired-relay solutions. The same can be said for any application using general-purpose programmable devices, such as microcomputers. If, however, a device could combine the function of motor starting with the function of overload protection such a device would prove useful and cost effective in many applications.
Overload relays are dedicated circuit protection devices designed to interrupt the flow of current in an electric circuit upon the detection of undesirable current levels over a period of time. Such current levels can lead to serious damage to a motor through the excessive heating of the motor windings. Upon detection of an overload condition, the overload relay outputs a trip command to a circuit opening mechanism such as a contactor, which disconnects the load from its power source. Many applications using motor starters also require motor overload protection. For such applications, an overload relay is typically connected into the motor starter circuit and housed in the same control unit enclosure containing the starter.
The most common overload relays are of the thermal type, which include a heater element through which the load current flows and a bimetallic strip that deforms as it is heated by the heater. If heated sufficiently, the bimetallic strip deforms to such an extent that it forces a contact open, which commands the contactor to open the motor circuit. One problem with thermal overload relays is that a supply of different heating elements must be maintained to adjust the relays for different load conditions. In practice, few, if any, extra heater elements are available when needed. Furthermore, any adjustments made are in discrete steps that depend on the ratings of the available heaters.
Electronic overload relays containing microcomputers, which measure the load current by means of current transformers and calculate the heating therefrom, avoid the adjustment problem of thermal devices, but at a greater cost. Even the low cost electronic overload relays, however, allow only a few parameters to be adjusted. Furthermore, the tripping characteristics of the electronic devices are designed to emulate the tripping characteristics of a thermal relay, instead of being designed to more accurately control the motor. To keep the cost of simple electronic overload relays competitive with the cost of thermal devices, inexpensive, low-performance microcomputers have been used. The cost of overload devices using higher-performance microcomputers cannot be justified for simple applications, therefore it would be desirable to combine the overload function with other needed functions.
In addition to providing a device for motor control, it would be advantageous to provide the device with the ability to accurately detect ground fault currents in the motor associated with the device, without causing spurious tripping during motor start times. In general, a ground fault in a motor results when an undesired current path is established to ground. For example, in a single phase motor the sum of the phase current and the neutral current should be zero. In a three phase motor, the sum of the three phase currents should be zero. Accordingly, when these sums are not zero a ground fault is assumed. During motor starting, it is not uncommon for transient phase imbalances to exist.